Changes in climate and snow cover and their synergistic influence on spring runoff in the source region of the Yellow River

As an important runoff-producing area, the runoff variation in the source region of the Yellow River (SRYR) has critical importance for the whole basin in broad aspects. In recent decades, the climate in the SRYR has undergone drastic changes, which affected runoff across different time scales. Many studies have focused on runoff in the SRYR with a long-time series, and presented a discordant relationship between precipitation and runoff. However, differences in this relationship over different time scales are ignored. Here, by using multi-source observation data and correlation analysis, climate elasticity, and principal component analysis methods, we document the changes in climate and snow cover and their synergistic influence on spring runoff. When the 20-year period was innovatively adopted, the runoff and precipitation coincided well during last three periods (1960-2019). The yearly precipitation presented a bimodal pattern, with the most significant increase in late spring and early summer. A bimodal pattern also appeared in annual runoff, and the rate of increase was much greater than that of precipitation (2.51%/year vs 1.01%/year). The runoff during main increase period (particularly in April) showed a high correlation with the remote sensing snow depth from November to March, but a poor correlation with snow depth from meteorological stations. Climate warming in the SRYR was much more reflected in minimum surface temperature (0.235 °C/year) than in air temperature minimum (0.081 °C/year) in last 20 years. However, the principal component analysis shows that the effect of temperature on spring runoff was obviously less than that of snow cover. A 1% variation in snow depth in the SRYR from November to March caused a 0.43% variation in runoff in April, and a 1% variation in snow days caused a 0.82% variation in runoff. This study will bring to light for understanding the evolution mechanism of spring runoff in the SRYR.